The present invention relates to an extruder for melting and extruding plastic granules of the type commonly used in the spinning of polymeric filaments.
In the extrusion of plastic granules, especially for a polymer that is suitable for use in a melt spinning process, the following process steps are usually involved; drying, feeding, melting, degassing and discharge of product. In the conventional systems the first process step, namely drying, is carried out by means of a drier and is generally performed independently of the actual extrusion, in a process that takes place continuously or also batchwise in a separate drying device. The spun-dry granules are then fed to an extruder in which the further process steps up to discharge of the product take place, the degassing being directly dependent on the degree of drying of the granules.
EP 0 134 977 B1 describes a device for removing volatile constituents from plastic materials, which for molding is conveyed through a plasticising screw and plasticised therein. This plasticising screw is arranged in a heated cylinder through which the particulate plastic material is fed. The plasticising screw comprises an actual plasticisation section and a conveying section arranged upstream of the latter. An additional conveying screw arranged upstream of this plasticising screw conveys the material to be extruded to a suction channel, from which it falls into the conveying section of the plasticising screw. Since the cylinder in which the plasticising screw operates is heated, and since through the work of the plasticising screw energy is supplied to the material to be extruded, volatile constituents escape, so that the granules are dried to a certain extent. These volatile constituents are removed, through the suction line, from the cylinder in which the plasticising screw operates. In order to ensure that these volatile constituents are removed, the conveying screw and the plasticising screw are driven so that only such an amount of granules is fed to the plasticising screw as to ensure that the latter is only partially filled in the region of the suction line, in other words in its conveying section.
As regards the efficiency of the drying of the granules, this known arrangement has inter alia the disadvantage that the granules are only indirectly heated. On the one hand energy losses thus occur, and on the other hand an additional device is furthermore necessary to ensure a reduced pressure for suctioning off the volatile constituents, which in turn requires additional energy. The known arrangement can therefore be used only to a limited extent for drying granules, and is better suited primarily for removing volatile constituents from the plastic material to be extruded.
It is accordingly an object of the present invention to provide an extruder that is of compact construction, which in addition to feeding, melting, degassing, and removing the resulting melt, can also effectively dry the plastic granules, and which furthermore is energy-saving and economical to operate.